are very good, and their drilling characteristics are considered excellent.
Acknowledgment Grateful acknowledgment is made to staff members of the Farm Machinery
Section, Agricultural Engineering Research Branch, who constructed the equipment; to staff members of the Division of Chemical Development, Tennessee Valley Authority, who supplied test materials; and to J. B. Breen of this laboratory, who assisted with the tests.
Literature Cited (1) Starostka, R. W., Norland, M. A., and MacBride, J. E., J. AGR. FOODCHEM.,3, 1022 (1955). Received for review Arovember 23, Accepted August 4, 1955.
7954.
S O I L CONDITIONERS
Dextran and Dextran Products as SoiI-Conditioning MateriaIs
1. J. NOVAK, E. E. WITT, and MALVERN J. HlLER The Commonwealth Engineering Co. of Ohio, Dayton, Ohio
The recent prominence given to dextran and its use as a plasma extender for humans, as well as the development of techniques for making dextran products, has suggested other commercial possibilities. The presence of dextrans in many soils and their stability to influence the binding of soil particles have been noted. Additional work is reported dealing with the effect of molecular weight ranges of dextran polymers on soil conditioning, including data on the influence of the structural branching of dextran polymers. Marked stabilization of soils, up to 60% increased plant seedling emergence, and up to 70% better crop yield, have been demonstrated with soils containing specific dextran products.
S
ANCIENT TIMES the plow has symbolized man’s agricultural efforts in tilling soils and improving their structure. Within recent times, however, it has become evident that natural agents in many soils help the farmer by imparting increased stability or permanence to soil aggregates. If this were not true, such soil aggregates would be broken down to a n undesirable degree mechanically, by tillage instruments, by the action of raindrops, or by excess moisture during rainy seasons. Without such natural stabilizing agents it would be necessary to expend much more than the currently estimated one-third available draft power on the farm ( 9 ) for plowing. Organic and inorganic soil colloids are believed to be largely responsible for the natural structural stability of many good soils. Unfortunately, much of the organic matter has been lost from many of our tillable soils. This continued depletion has focused attention on the development of synthetic materials, which stabilize and condition soil aggregates. Such synthetic materials, howei’er, have not been considered complete substitutes for natural organic materials such as stable manure, nor are they economically feasible from the farmer’s viewpoint. Long before man recognized the need for tillage and the addition of plant nutrients to the soil and before the term “soil conditioner” was coined, Nature supported and sustained plant and vege-
INCE
1028
table life, by establishing a relationship between plant nutrients taken from the soil and those returned to it. Soil nutrients taken in water-soluble forms by plants are returned to the soil as organic material-dead plant and animal matter. This organic material is attacked by the multitudinous soil microorganisms and converted to forms again available to plants, and to a product of a mucilaginous nature called humus, I t is this material, composed in part of bacterial polysaccharides (3-5, 8, 9, 7 4 , which is mainly responsible for the desirable, well drained, granular condition of typical virgin soils. Dextrans, synthesized by a number of soil bacteria ( 8 ) , comprise one of these bacterial polysaccharidal groups. Studies on soil-water extracts, particularly rich in organic matter, demonstrate that water-soluble, alcohol-precipitable polysaccharidal substances may be present. Certain investigators have reported the influence of bacterial polysaccharides on soils (3-6), but the use of bacterial dextran polysaccharides as soil conditioners has not been sufficiently investigated. Some properties of dextran were clarified in 1869 by Scheibler (77) in Germany. Dextran is now known to be a polysaccharide-type polymer made up of many molecules of glucose in long chains, having definite repeating linkages and branchings, and chemically related to glycogen. Dextrans are produced from bacteria
AGRICULTURAL AND FOOD CHEMISTRY
in typical fermentation media, which may contain such materials as corn steep liquor, peptone, and inorganic salts, or may be made in cell-free media from a n exocellular enzyme termed dextran sucrase. Such enzymes can be concentrated so that, when added to strong sucrose solutions, they will rapidly form enough dextran to prevent perceptible flow in the system when the container is inverted. Leuconostoc mesenteroides NRRL-B512, produces the native dextran used for the manufacture of blood plasma volume expanders. Dextrans, as they are biosynthesized, may have molecular weights in the millions and yet many can be dissolved in water to form colloidal solutions of high viscosity. Few, if any, related materials possess such high molecular weights with concomitant water solubility. The solubility of native dextrans is related to their molecular structures. Those having a high ratio of alpha-1,6 to non-l,6 repeating anhydroglucose unit linkages may be more water-soluble than those with a low ratio; although some dextrans are very insoluble in water, they are apparently all swollen when added to water. Physical and chemical properties such as water solubility, viscosity, molecular weight, and structure vary widely among different types of dextrans. The number of dextran-producing organisms is large and only a few of the products made by these organisms have been in-
vestigated. At least 96 strains of bacteria may produce glucose polymers that vary in their properties (7). During the past two decades, The Commonwealth Engineering Co. of Ohio has worked with dextran and dextran products. I t has developed many uses for these materials, as well as a process for producing a blood plasma expander for the treatment of shock in humans. Because of the reported presence of dextrans in soil and their influence on retention of soil moisture, discovered in preliminary experiments, it was believed that some dextrans might serve as suitable soil-conditioning materials. A series of laboratory investigations was begun in 1952. Prior to 1954 the term “soil conditioner” was insufficiently defined. The situation was clarified by the Federal Trade Commission (2), which defined a soil conditioner as “any synthetic organic chemical substance, or chemically modified natural substance, which is represented as having a primary function of forming and/or stabilizing soil aggregates in soil to which it has been applied and thereby improving the resistance of such soil to the slaking action of water, increasing its water and air permeability, improving the resistance of its surface to crusting, improving its ease of cultivation, and/or otherwise favorably modifying its structural or physical properties.” Objective
.4n extensive program was undertaken to investigate dextran products for use in the conditioning or stabilization of soils, to obtain information on the influence of dextran-treated soils on seedling emergence, plant growth, and crop yield, and to ascertain the relative differences in activity of various types of dextran polymers with different branching structures and molecular weight ranges. If particular dextran products were found to have significant initial soilconditioning activity, preliminary information would be obtained o n the durable or transitory action of such dextran products for soil conditioning. Test Procedures
Soil Samples. T o study this problem adequately, several classes of soil were obtained for evaluating dextran as a sui1 conditioner. Soils were chosen on the basis of their “need” for conditioning and their content of sand, silt, and clay particles. Sandy loam, silt loam, and clay loam soil classes were selected to obtain satisfactory ranges of soil particle size (9, 73). Soil Mechanical Analyses. Mechanical analyses of soil for sand, silt, or clay
content were made as described by Puri ( 70). Moisture Content. Soil moisture content was determined by drying weighed samples a t 105 to 110’ C. for 24 hours in tared aluminum weighing dishes. Air-dried silt Procedure for loam soil was Establishing Period passed through of Effectiveness a 20-mesh sieve and divided into three groups, each consisting of three 300-gram portions, which were placed in jars and covered with loosely fitted lids. I n two groups 0.5% dry powdered dextran 60 or 21 was thoroughly incorporated. The third group served as controls. The moisture content of each sample was brought to 20%, and then the samples were covered with loosely fitting lids and placed in an incubator set a t 37’ C. During incubation the soil in each container was aerated daily by removing the lids and slowly rotating the containers. The moisture content of all samples was kept between 10 and 207,. When it had diminished to 1OyGin a sample, water was added to bring soil moisture content to 20’30. At periodic intervals, a wet-sieve soil stability test was run on each sample. Soils were passed Method for through a 4-mesh Incorporating sieve and divided Dextran into Soils into the DrODer I
1
number of weighed samples, and moisture was determined. T h e powdered, dry dextran product was then thoroughly mixed with the required soil samples, and placed in containers. Water was added to give a 30YGmoisture content. The samples were allowed to stand and were weighed a t periodic intervals until weight loss showed the samples to contain 15% moisture. Appropriate seeds were then planted in this soil. The preliminary basis for evaluating the effect of dextran on laboratory soils was measurement of the stability of the soil aggregates under the influence of excess moisture. A modification of the Wet-Sieve ‘Oil wet-sieving method Stability Test described by Bryant, Bendixen, and Slater ( 7 ) was used. Procedure. All soil samples were prepared for treatment by sieving through a 20-mesh screen. Appropriate weighed amounts of powdered dextran were added to 100-gram samples of the soil and thoroughly mixed. T h e moisture content of the soil was then brought to 30%, based o n the weight of dry soil. T h e wet soil was allowed to stand for 24 to 48 hours, when it was passed through a 4-mesh screen. Twentyfive-gram samples of soil aggregates, with diameters less than 4.7 mm. and greater than 2.0 mm., were used for determining water stability. Water stability of the soil under test was re-
corded as that percentage of dried soil aggregates, greater than 0.25 mm., retained on the screens after wet-sieving for 30 minutes. Brookston silty clay loam Greenhouse soil was passed through a Tests 4-mesh sieve. One-gallon stone jars were filled with 3000 grams of this prepared soil, 0.570 of dextran based on the oven-dry weight of the soil was added and thoroughly mixed, and the technique described above was followed. T h e surface soil in each j a r was stirred with a spatula and ten beans were planted in each jar. All jars were placed so that they received a n equal amount of light. Four days after planting, the sFrouted beans were counted and the number was recorded as seedling emergence data. -411 jars containing bean plants were periodically watered to maintain a moisture content of approximately 2070. Fifty-four days after planting, the plants in each jar were harvested by cutting off at the soil level and dried for 60 hours a t 120’ F. in a mechanical convection oven. Tests were carried out on a n eroded phase of soil similar to Tests the Russell silt loam in Greene County, near Xenia, Ohio. Dextran 60 of 40- to 100-mesh was incorporated in this soil to a depth of about 4 inches. Its conccntration in the soil was calculated to be 0.37,. A garden fertilizer (Vigoro) ivas then applied a t a rate of 2 pounds per 100 square feet. Soybeans were drilled solid and grown in triplicate plots, which were staggered to reduce error from variation in soil fertility. Sweet corn was planted bv hand in rows 3 feet wide with seeds spaced 8 inches apart in the row. Soybean crop yield was determined 88 days after planting by cutting down the plants in the plot with a scythe and drying as described under greenhouse tests. I
Experimental Findings
I n order to deInfluence Of Dextran termine whether on Stabilization or not dextran has any effect on certain soil classes and to establish its range of soil-stabilizing influence as well as the concentration required, a series of experiments was carried out. I t was established that dextran stabilizes different classes of soils to different degrees and that varying percentages of a dextran, control soil stabilization in some ratio to the amount added. Table I presents a typical set of determinations resulting from the waterstability test with dextran 60-stabilized soils. T h e data show that dextran is effective in stabilizing certain classes of soil from the coarse-textured through the fine-textured ones. I t is particularly interesting to note the stabilizing effect
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Table 1. Effect of Native Dextran 60 Concentration on Stabilization of Soils as Determined by Wet-Sieve Soil Stability Test Dextran,
Stability,
Sail Class
%
%
Clav Gam
0.0 0.3 0.6
Silty clay loam
0.0 0.3 0.6 0.0 0.3 0.5 0.0 0.5
42.2 69.8 83.4 61.7 78.2 78.6 29.3 53.0 98.4 33.0 76.2
Silt
loam Sandy loam
0.3% dextrans differing in molecular structure is given in Table 11. These dextrans all had molecular weights above 1,000,000. Tests represent averages of a t least duplicate analyses.
Table II. Effect of Native Dextrans Differing in Molecular Structure on Increase in Stabilization of a Clay loam Soil
Treatment
Ratio of Dextran Molecular Anhydroglucose 1,6 linkages fo Non- I ,6 linkages
Control Dextran 61 Dextran 21 Dextran 60
on the sandy loam. Both soil classes are imuroved bv addition of dextran. The molecular Influence of structure of naMolecular Structure tive dextrans of on Stabilization different types varies widely. I t was of particular importance early in this work to establish which types were most efficient for practical soil stabilization. Using test procedure described above, certain facts were established for later use in the program. A typical set of findings showing the degree of stabilization of one class of soil produced by I
_
3to 1 8 to 1 18 to 1
Soil Stability,
%
51.5 62.8 65.4 68.5
The results show that a native dextran with a high ratio of 1,6 to non-1,6 anhydroglucose linkages may be superior as a conditioning agent for this particular soil. Such dextrans are more water-soluble and in forming colloidal solutions they are not influenced by differences in solution p H as much as less soluble dextrans with lower ratios of 1,6 to non-l,6 anhydroglucose linkages. Dextrans such as Type 61 may, however, form true colloidal solutions at p H values above 7.0. In the presence
of constant soil moisture content, the soil p H (or free acid or alkali content) may therefore be related to the degree of soil stabilization obtained for a particular dextran product. The p H of the soil used in these experiments was determined to be 6.0. The results, therefore, substantiate the belief that acid soils may be stabilized more efficiently with the more watersoluble dextrans having higher ratios of 1,6 to non-l,6 anhydroglucose linkages. . I t was of imporInfluence of Molecular Weight tance to establish any relationships Ranges on between the moStabilization lecular weight ranges of a suitable dextran and increased stability for particular soil classes. During the last decade considerable progress has been made toward the preparation of dextrans having particular molecular weight ranges. This has been due principally to improved fractionation techniques and methods for determining molecular weight of such “polyglucoses,” either before or after controlled acid hydrolysis (72). Water-soluble dextran 60 was chosen for these experiments. By acid hydrolysis, and/or selective fractionation with methanol and drying, pure dextran 60 samples were obtained with weight average molecular weight ranges by light scattering of 3 X IO4> 20 X lo4, and
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AGRICULTURAL AND FOOD CHEMISTRY
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Table 111. Influence of Storoge Time at 37" C. on Silt l o a m Soil Stabilized with 0.5% Dextran
% Stability
Treatment
o f Silt loam Soil o f f e r 3 days 10 days 51 days
Control Dextran 60 Dextran 21
33.6 53.0 57.8
39.2 74.4 75.4
51.7 81.2 89.7
Table IV. Stability of Brookston Silty Clay l o a m in Greenhouse Work 69 Days after Treatment with 0.5% Dextran Type
Control Dextran 60 Dextran 21
Stability,
%
70.0 93.3 91.5
over 1 X 106. Such samples were incorporated into three classes of soils according to the laboratory test procedures given above. The results shown in Figure 1 indicate that dextran 60 with a weight average molecular weight range of 3 X lo4 or greater increases the stabilit) of the three soils tested. Dextran 60 with a low weight average molecular weight range of 3 X lo4 is more effective for increasing the per cent stability of the coarser textured sandy loam than of the silt loam and possiblv the clay loam soils. A weight average molecular weight range greater than 20 X lo4 does not significantlv increase the effectiveness of dextran 60 for stabilizing the soils tested. The results indicate that as the soil particle size range decreases. the percentage increase of soil stability may be less for dextran 60 of low weight average molecular weight. As the weight average molecular weight range increases from 3 X lo4 to 20 X lo4, the soilstabilizing effectiveness of the dextran 60 becomes greater for the finer textured soils tested. Although it is of secondary Influence importance that a condiof Time tioning agent remain effective for a number of seasons or years, because of the necessary tillage and cultivating activity, a good conditioner will reduce the necessary periodic tillage. Good economics should be applied and the cost of one conditioning treatment be divided by the number of years a conditioner remains effective. It was therefore of interest to determine whether dextrans have a durable or a transitory influence on the stabilization of soil. T o answer this question a series of studies along these lines was initiated, using the test procedures described. Table I11 presents a typical stability report for the treated and untreated soil
a t 3, 10, and 51 days after treatment. Results show that the stabilizing influence of dextran on the silt loam soil aggregates is not a transitory phenomenon, as might be anticipated if the added dextran had been destroyed by the microorganisms present. As it has been demonstrated (Figure 1) that the reduction in molecular weight of added dextran 60 from over 1 X 106 to 20 X 1O4 does not significantly reduce the stability of the three classes of soil studied, it can be assumed that, even if particular microorganisms slowly attack particular dextrans in a treated soil, considerable time could ensue before dextran degradation would cause a reduction in the soil stabilization initially attained. As a further check on the permanency of dextrans in soil, this series of experiments was carried over into the greenhouse work. Table IV reports on a Brookston silty clay loam treated with 0.5% dextran for use in growing soybeans in the greenhouse. This soil still had superior stability over the control 69 days after treatment. It may be concluded that added dextran increases the stability of soil aggregates for a t least one growing season in temperate climates. The total effect of this soil-stabilizing activity over a number of growing seasons remains to be determined. Influence on Influence of DextranPlants. -4fter Treated Soils laboratorv data and information on the character of dextrans and their utility in conditioning soils had been secured, it was necessary to consider the influence of dextran products as soil stabilizers on the growth of plants. A series of tests on soybeans and mung beans in the greenhouse was undertaken. The procedure for controlled growth and test conditions is outlined above. The soil used for greenhouse tests was slightly acid (pH 6.58). Chemical analysis showed that available phosphorus was high (105 pounds per acre), while potassium was low (89 pounds per acre). The Soil Conservation Service classes this soil as a Brookston silty clay loam. .4nalysis showed this soil to be 55.6y0 water stable. Typical growth and yields of crop from these greenhouse tests are presented in Table V. The figures reported are the average of triplicate tests. Four days after planting more beans had sprouted in the dextran-treated soil. Prior to harvesting the plants from the treated soil were larger and superior in appearance to the controls. The soybean plants were harvested 54 days after planting, a t the hay-cutting stage. The bean pods had formed, but the plants had not completely matured. The consistent increase of plant yield up to 70% for soybeans and 78.9y0 for mung beans reflects the marked in-
fluence of dextran in soils on plant growth. Figure 2 shows a typical improvement in soybean seedling emergence after soil treatment with 0.5% dextran 60. Influence on Crops. As dextran was shown to increase soil stability in laboratory experimentation, to improve plant seedling emergence and plant yield in controlled greenhouse tests, and to resist bacterial degradation, it was decided to initiate limited field tests on plots in Greene County, near Xenia, Ohio, to obtain additional information of possible practical value on dextran products for soil conditions. T h e pH of the soil used was 5.75. .4nalysis showed that it contained 42 pounds per acre of available phosphorus ( P 2 0 5 ) and 166 pounds of potassium ( K 2 0 ) . The wet-sieve stability of this soil was 73%, as determined by wetsieve analysis. The relatively high stability is attributed mostly to the effects of a great deal of green plant material plowed under in the spring. The dextran 60 was a 40/100 mesh, light tan, free-flowing powder which did not cake when stored for several weeks in air-tight fiber drums. It flowed freely and evenly from the mechanical distributor onto the soil. Just before seeding, the wet sieve stability of the soil was determined to be 90%. Observations were made periodically on seedling emergence, crop appearance, and crop yield. Some of the typical data are produced in Table VI. Figure
Table V. Crop Yields (Dry Basis) of Beans Grown in Greenhouse Brookston Silty Clay l o a m Treated with Native Dextran
Treotmenf
Average Plant Amount, Weight, % Grams
Increase in Yield,
%
Soybeans Control Dextran60 Dextran 21
0.5 0.5 0.5
... 25
2.0 2.5 3.4
70
Mung Beans Control Dextran Dextran Dextran Dextran
... 60
60 61
21
0.109 0.160 0.170 0.190 0,195
0.3 0.6 0.6 0.6
...
46.8 56.0 74.3 78.9
Table VI. Corn Seedling Emergence 20 Days after Planting on Field Plots in a Silt l o a m Soil Treated with 0.3% Dextran 60 No. of Plants per Control Row
Plants per Dextran Row
15 12
20 15
12
No. o f
16
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lncrease in Plant Emergence,
44 25 33
1031
%
Figure 2.
Emergence of soybean seeds in untreated and dextran-treated silty clay loom soil, 4 days after planting left.
Untreated, 50% seedling emergence.
Right.
Treated wilh dextran 60, 80% seedling emergence
3 is a photograph of two of the test serve as source of plant nutrients. I n the work heretofore described, however, no rows of corn taken 34 days after planting. The left row is the control. An average fertilizer elements were added to the increase of 16.5y0 in total yield of soynatural dextran products used in the bean plants a t the hay-cutting stage Figure 3. Influence of 0.3% native dextran 60 on was observed from growth of sweet corn the dextran-treated plots by comparison with the control plots.
Discussion The preceding results, as well as other work on dextrans conducted over the past few years, indicate certain advantages when dextrans are used for soil conditioning. Dextrans are produced by hiosynthetic fermentation processes which require particular hut inexpensive compounds rich in nitro. gen, phosphorus, and potassium for maximum production yields. The principal raw material - sugar - is cheap, in free supply, and available in tonnage quantities. Dextran products which can be economically produced not only function as soil conditioners but also contain appreciable amounts of nitrogen, phosphorus, and potassium, which then may also
1032
AGRICULTURAL AND FOOD CHEMISTRY
laboratory, greenhouse or field tests. Dextrans are natural substances, and are nontoxic to plants grown in soils containing them. Undesirable overstabilization of the soil is not easily accomplished. A range of dextrans can he produced which vary considerably in their stahilizing effects a n a variety of soil classes. Results of experimental work indicate that a correlation among soil texture, dextran molecular weight, and dextran structure undoubtedly exists. It suggests that nne dextran might be used for a soil class such as a sandy loam. another for silt loam, and yet another for clay soil, far maximum effectiveness. The dextran products described are free flowing, relatively dense, finely divided powders which are easily applied to the surface of the soil, and can be incorporated in it by the use of conventional farm implements.
Conclusions Certain dextrans when added to test sails, in concentrations not exceeding 0.6%, have increased the wet-sieve stability of the soils Studied, per cent seedling emergence, rate of plant growth and crop yield for the plants used. Experiments have been successfully concluded on soybeans, mung beans, and corn. A relationship is indicated among dextran type, dextran structure, dextran molecular weight, and the degree of soil stabilization achieved for particular soil classes. The over-all work on which this paper is based may serve as an interesting new wedge for examination of the effects of of molecular structure and molecular weight relationships of polymers on the properties of soils. T h a t there is a definite influence on the binding of different particle sizes in soils by the
various dextrans should prompt other investigators to examine this action further. This paper has been concerned primarily with studies leading to the relative effects produced and not to their causes.
Acknowledgment The authors wish to express their appreciation to H. A. Toulmin, Jr., and to The Commonwealth Engineering Co. of Ohio for permission to publish their findings in this series of studies.
Literature Cited (1) Bryant, J. C., Bendixen, T. W., and Slater, C. S., Soil Sci.,65, 341-5 (1948). (2) Federal Trade Commission, Wash-
ington, D . C., “Trade Practice Rules for the Chemical Soil Conditioner Industry,” Oct. 15, 1954. (3) Geoghegan, M . J., and Brian, R. C.. Biochem. J.., 43., 5-13 (1948) .’ (4) Geoghegan, M . J., and Brian, R . C., h’ature (London). 158, 837 (1946). (5) Haworth. W. N., Pinkard, F. W., and Stacey, M., Zbid., 158, 836-7 (1946). (6) Hedrick, R. M., and Mowry, D. T., Soil Sci., 73, 427-41 ( 1952). ( 7 ) Jeanes, Allene, Haynes, W. C.. Wilham, C. A., Rankin, J. C.. Melvin, E. H., Austin, M . J.. McCluskey, J. E., Fisher, B. E., Tsuchiya, H. M., and Rist, C. E., J . Am. Chem. Soc., 76, 5041 (1954).
(8) Martin, J. P., Zbid., 59, 163-74 (1 945). (9) Millar, C. E., and Turk, L. M., “Fundamenta!s of Soil Science,” 2nd ed., Wiley, New York, 1951. (10) Puri, A. N., “Soils, Their Physics and Chemistry,” Reinhold, New York, 1949. (11) Scheibler, C., Z. Ver. deut. ZuckerZnd., 19, 472 (1869); 24, 309 ( 1874). (12) Senti, F. R., and Hellman, N . N., Abstracts of Papers, 125th Meeting, ACS, p. 8D, 1954. (13) U. S. Dept. Agriculture, Washington 25, D.C., Agricultural Handbook 18, “Soil Survey Manual,” 1951. (14) Waksman, S., “Soil Microbiology,” Wiley, New York, 1952. Received for review January 27, 1955. Accepted October 21, 1944.
P E P P E R M I N T OIL VERNON K. WATSON and J. 1. ST. JOHN
Relation of Maturity and Curing of Peppermint Hay to Yield and Composition of Oil
Washington Agricultural Experiment Stations, State College of Washington, Pullman, Wash.
The yield of peppermint oil per acre is higher in the lower Yakima Valley of south central Washington than in any other known area. Peppermint plants bloom profusely and give maximum yield at time of full bloom. Oils contain an appreciable portion of blossom oil, high in menthofuran, and some practical method is needed to reduce the number of blossoms at harvest time. Retiming of irrigation and fertilizer application and clipping of blossoms might be of value. Harvest at an early stage of plant development might improve odor and flavor, but at the expense of total yield. Total menthol and esters increase with plant development. The optical rotation of the oil may or may not decrease to a minimum at the time of maximum yield. Length of curing after cutting has only a minor effect on composition.
P
OIL is obtained by the steam distillation of the peppermint plant (Mentha p$erita). T h e Pacific Northwest is the source of about 60% of the oil produced in the United States; about 21% comes from the state of Washington (7). Peppermint is grown on muck soils in the midwestern states of Michigan and Indiana, and on somewhat lighter soils in the states of Washington and Oregon. There are two main areas in the West which. according to the buyers, produce somewhat different peppermint oils, although the same plant, Mentha piperite. is grown in both areas, as well as in the Midwest. In the older production area in Washington and Oregon, west of the Cascades in the Willamette and lower EPPERMIST
Columbia valleys, peppermint is grown on heavy alluvial soils which are high in organic matter. There is occasional irrigation during the dryer parts of the growing season. T h e peppermint oil produced there is somewhat similar to that produced in the Midwest. The second area, located in south central Washington, is in the lower Yakima Valley and along the Columbia River immediately below its junction with the Yakima River. Peppermint is grown on mineral soils that have developed under semiarid conditions, in a n environment characterized by wide temperature fluctuations and more hours of sunlight of much higher intensity than in the other areas. Cultural conditions and practices in south central Washing,
V O L . 3, ND.
ton may vary from those in other areas. This is the only known area growing peppermint entirely under irrigation. Irrigation is principally by furrow, but sprinklers are used to some extent. There has been a rather rapid increase of peppermint oil production in this area. T h e yield of oil per acre in south central Washington is over three times that obtained in the Midwest and almost twice that of Oregon and western ivashington. Plant diseases and insect damage have decreased both acreage and yield in Indiana and Michigan. Rust has become a problem in Oregon and western Washington. As yet the lower Yakima Valley in Washington is free of these plant diseases and has only a small infestation of the mint flea beetle. 12, D E C E M B E R 1955
1033
